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Surface elevation table (SET) monitoring at Assateague Island National Seashore.

Figure 1: Surface elevation table (SET) monitoring at Assateague Island National Seashore.

Background

Coastal wetlands exist within a narrow range of elevation which is influenced by local hydrologic conditions. As sea level rises, coastal wetland systems must build elevation at a rate equal to or greater than the rate of sea-level rise to maintain favorable hydrologic conditions for their survival (Figure 2). With global rates of sea-level rise predicted to increase over the coming decades (Church et al. 2010), many more coastal wetlands may become vulnerable to increased inundation. Additionally, certain areas of the coast, such as the mid-Atlantic region, can undergo even greater sea-level rise due to changes in coastal ocean circulation (Sallenger et al. 2012).

The Northeast Coastal & Barrier Inventory and Monitoring Network (NCBN) uses the SET-MH technique (surface elevation table-marker horizon) to monitor elevation change as a vital sign of salt marsh health at Assateague Island National Seashore. In the following sections, we use these data to answer the following questions:

  1. What are the elevation characteristics of coastal wetlands in Assateague Island National Seashore? (Figure 2A; Results Section 1)

  2. How do surface and subsurface processes control surface elevation change at Assateague Island National Seashore? (Figure 2B; Results Section 2)

  3. How do local wetland elevation dynamics relate to the local hydrology (e.g., local tidal datums)? (Figure 2C; Results Section 3)

  4. Is elevation gain sufficient to keep pace with current and predicted sea level rise? (Figure 2D; Results Section 3)

Conceptual diagram showing the relationship among wetland surface elevation (A), tidal datums (C), and their respective rates of change (B and D). Understanding these relationships is important for evaluating the sustainability of a wetland community to sea-level rise.

Figure 2: Conceptual diagram showing the relationship among wetland surface elevation (A), tidal datums (C), and their respective rates of change (B and D). Understanding these relationships is important for evaluating the sustainability of a wetland community to sea-level rise.

Methods

Protocols (Vital Signs)

NCBN’s long-term, standardized monitoring protocol for monitoring surface elevation change (Lynch et al. 2015) is implemented semiannually in the spring and fall at Assateague Island National Seashore. Changes in surface elevation are measured using the SET-MH technique (surface elevation table (SET) – marker horizon (MH)):

  • The SET is a portable mechanical leveling device providing repeated, high-resolution measurements of elevation change in wetland sediments or shallow water bottoms relative to the depth of a permanent benchmark that has been anchored into the soil until refusal. During measurements, the SET arm is attached to the permanent SET benchmark and extended over the marsh surface at four fixed positions. The SET arm is carefully leveled to rest horizontally to the ground, and each of nine fiberglass pins are lowered through the arm to the soil surface. The height of each pin above the arm is measured on repeated sampling events. Changes in the height of the pins between sampling events are used to quantify soil surface elevation change over time relative to the permanent benchmark.

  • Marker horizons are artificial soil layers (e.g., feldspar) established on the surface of wetland or shallow water bottoms to measure subsequent surface sediment accretion. Cores are taken with a soil corer from the soil surface to this layer on repeated sampling events, and the thickness of the sediment accumulated above the layer is measured as vertical accretion.

Sample Size

Surface elevation change is actively monitored at four sites within Assateague Island National Seashore (Figure 3). A fifth site, Pine Tree, was monitored between 2016 to 2022 as part of a pilot effort to compare rates of surface elevation change derived from SETs to rates derived by other elevation measurement methods (Lynch et al. 2024). Additional details on the status of each site, the number of replicate SET-MH stations at each site, and the dates of the initial and most recent measurements are shown in Table 1 below.

All data utilized in this report are available for public use and can be downloaded from the NPS DataStore.

Table 1: Surface elevation table sites monitored by NCBN within Assateague Island National Seashore.
Site SET-MH station count First reading Most recent reading Status
Marsh 11 4 2009 2024 active
Marsh 5 (Pope Bay) 4 2009 2024 active
Marsh 6 (Pine Tree) 4 2009 2024 active
Marsh 8 (Valentines) 4 2009 2024 active
Pine Tree Study 3 2016 2022 inactive

Figure 3: Individual SET stations within Assateague Island National Seashore. Click on the points to see the site and station labels.

Results

1. What are the elevation characteristics of coastal wetlands at Assateague Island National Seashore?

Across all four actively monitored sites at Assateague Island National Seashore, the park-wide average rate of surface elevation change was 4.14 ± 0.72 mm/yr. Site-level rates of surface elevation change varied from a low of 2.86 ± 0.23 mm/yr at Marsh 5 (Pope Bay) to a high of 5.51 ± 0.19 mm/yr at Marsh 8 (Valentines) (Table 2; Figure 4).

Table 2: Site-level rates of surface elevation change within Assateague Island National Seashore. Symbols next to surface elevation change rates denote the model p-value, where * indicates the rate is significantly different from 0, and · indicates the rate is not significantly different from 0.
Site Rate of surface elevation change (mm/yr) Adjusted r2 Date range
Marsh 11 2.92 ± 0.11* 0.97 2009-2024
Marsh 5 (Pope Bay) 2.86 ± 0.23* 0.86 2009-2024
Marsh 6 (Pine Tree) 5.27 ± 0.20* 0.97 2009-2024
Marsh 8 (Valentines) 5.51 ± 0.19* 0.97 2009-2024
Pine Tree Study 3.21 ± 0.29* 0.95 2016-2022

Figure 4: Site-level surface elevation change at Assateague Island National Seashore. Rates of surface elevation change (SEC) are shown in panel labels. Hover the mouse over a data point to see the cumulative surface elevation change value on a specific date, or click and drag over a specific time to zoom in. Double click to zoom back out

2. How do surface and subsurface processes control surface elevation change at Assateague Island National Seashore?

Across all four actively monitored sites at Assateague Island National Seashore, the park-wide average rate of vertical accretion was 4.80 ± 0.43 mm/yr. Site-level rates of vertical accretion varied from a low of 3.70 ± 0.22 mm/yr at Marsh 11 to a high of 5.77 ± 0.30 mm/yr at Marsh 8 (Valentines) (Table 3; Figure ).

The park-wide average rate of subsurface elevation change was -0.66 ± 0.45 mm/yr. Site-level rates of subsurface elevation change varied from a low of -1.85 at Marsh 5 (Pope Bay) to a high of 0.25 at Marsh 6 (Pine Tree) (Table 3; Figure ).

Table 3: Site-level rates of surface elevation change, vertical accretion, and subsurface elevation change within Assateague Island National Seashore. Subsurface elevation change represents the difference between surface elevation change and vertical accretion. Symbols next to surface elevation change and vertical accretion rates denote the model p-value, where * indicates the rate is significantly different from 0, and · indicates the rate is not significantly different from 0. “NA” indicates that vertical accretion was not measured at a specific site.
Site Rate of surface elevation change (mm/yr) Rate of vertical accretion (mm/yr) Subsurface elevation change (mm/yr)
Marsh 11 2.92* 3.70* -0.78
Marsh 5 (Pope Bay) 2.86* 4.71* -1.85
Marsh 6 (Pine Tree) 5.27* 5.01* 0.25
Marsh 8 (Valentines) 5.51* 5.77* -0.26
Pine Tree Study 3.21* NA NA